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Compositional, Structural and Phase Relationships in Titanian Ixiolite And

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Compositional, Structural and Phase Relationships in Titanian Ixiolite And 547 The Canalian M ineralo g ist Vol.36, pp. 54'1-561(1998) COMPOSITIONAL,STRUCTURAL AND PHASE RELATIONSHIPSIN TITANIAN IXIOLITEAND TITANIAN COLUMBITE. TANTALITE PETR EERNfI Depanment of Geological Sciences, University of Manitoba, Wnnipeg, Manitoba R3T 2N2 T. SCOTTERCIT Research Division, Canadian Museum of Nature, Onawa, Ontario KI P 6P4 MICHAELA. WISE Department of Mineral Sciences,Smithsonian Institution, Washington,D.C.20560, U.S.A. RON CHAPMAN ANDHARVEY M. BUCK Departtnent of Geological Sciences, University of Manitoba, Wnnipeg, Manitoba R3T 2N2 ABSTRACT Titanium-enriched members of the columbite family (orthorhombic) commonly contain exsolved niobian-tantalian rutile, or are aggregatedwith a coprecipitated rutile phase.This assemblageis the Fe,Mn,M,Ta-rich counterpart of the Ti-dominant niobian rutile + exsolved titrnian columbite pair. The rutile phase is enriched in Fe2*,T4 Fek and Sn, whereasthe orthorhombic phase favors Mn, M, Sc, W and Zr. Most samples of the orthorhombic phase are defined as titanian columbite - tantalite; they have low to moderate Ti content, moderately to considerably disordered structure, and convert to highly ordered columbite - tantalite on heating. A minor number of samples of the orthorhombic phase develops the highly ordered structure of wodginite on heating, which identifies them as titanian ixiolite, the disordered counterpart of titanowodginite. These samples are highly disordered in the natural state.They are Ti-rich, with Mn > Fe and Ta > Nb; these features correspond to the composition of natural titanowodginite and of the only titanowodginite synthesizedto date. However, subordinatebut significant Sn seems to be present in all samples of titanian ixiolite as well as natural titanowodginite, and it may affect the path of ordering. Excess of Ta over the (Fe, MnXNb, Ta), stoichiometry of columbite, characteristic of most compositions of wodginite, may play a significant role. Ordered nuclei potentially present in the bulk of natural disordered phasesalso may be important. Keryords: titanium, columbite, tantalite, ixiolite, order-disorder, exsolution. Souuene l,es memb'resdu groupe de la columbite (orthorhombique) qui sont enrichis en titane possbdentg6ndralement des domaines de rutile enrichi en Nb et Ta dus tr l'exsolution, ou bien des agr6gatsde rutile copr6cipit6. Il s'agit de I'assemblage contenant Fe, Mn, M et Ta qui est la contrepartie de I'association rutile niobifere + columbite titanifBre exsolv6e. La phase rutile est enrichie psz*, Ta, Feh et Sn, tandis que la phase orthorhombique favorise Mn, Nb, Sc, W et Zr. Dans la plupart des cas, la phase orthorhombique"o fait partie de la s6rie columbite - tantalite titanifdre; elle possbdeune teneur faible d mod6r6e en Ti et une structure relativement d6sordonn6e,et se transforme en columbite - tantalite bien ordonnde en chauffant. Dans un nombre restreint de cas, la phase orthorhombique se transforme en wodginite bien ordonn6e en chauffant, ce qui les identifie comme ixiolite titanifdre, la contrepartie d6sordonn6e de la titanowodginite. Ces 6chantillons sont foftement d6sordonn6s i l'6tat natuel. Ils sont riches en Ti, avec Mn > Fe et Ta > Nb; ces caract6ristiquescorrespondent h la composition de la titanowodginite naturelle et du seul exemple qui en ait 6t6 synthdtis6jusqu'd maintenant.Toutefois, il semble que des quantitds non n6gligeables d'6tain soient pr6sentesdans tous les 6chantillons d'ixiotite titanifdre et de titanowodginite naturelle, et ceci pourrait influencer la mise en ordre. Un exc6dent de Ta par rapport I la stoechiom6trie Fe,MnXNb,Ta)2 de la columbite, qui caract6risela plupart des compositions de wodginite, pourrait aussi jouer un 16le important. Les domaines ordonn6s potentiellement pr6sents dans les phasesnaturelles d6sordonn6espourraient aussi €tre importants. (fraduit par la R6daction) Mots-cI6s: titane, columbite, tantalite, ixiolite, ordre-d6sordre, exsolution. t E-mail address: [email protected] 548 INTRoDUCTIoN Unit-cell dimensions are provided for the natural phase by Sveshnikova et at. (1965) and dernf er aI. (1979), Ixiolite was redefinedby Nickel et al. (1963) as a and are complemented by data on heated material by complex oxide of the @,B)O, type, a Sn-substituted, Paul (1984), Ferreira (1984) and Cern! et at. (1989). cation-disorderedderivative of the 4820.6columbite The above sources suggest that the Ti-enriched structure (in which A = Fe, Mn, Mg, and B = Nb, Ta). orthorhombic minerals of the ABrOuor (A,B)O, types More specifically, ixiolite was assignedthe composition are either homogeneous,or closely associatedwith (Ta,Nb,Sn,Fe,Mn)Or,with an orthorhombic unit-cell a rutile phase. They show variable degrees of cation corresponding to that of disordered columbite; on disorder, and attain on heating either an ordered heating, it was reported to transform into a presumably columbite structure or a wodginite structure. However, orthorhombic "olovotantalite'Llike phase. the data are too limited for any attempts to quantify and "Olovotantalite" (Matias 196l) was later identified generalizethese relationships and their controls. with monoclinic wodginite (Gorzhevskayaet al. 1974), and further study of samplesof Sn-rich ixiolite from Savpr-ns Exavwep numerous localities established its ordering upon heating to wodginite as a rypical feature flilise & eernf Table I provides condensedinformation on the 1986,Wise 1987).The presenceof a slight monoclinic sources ofdatafrom the literature, and on the samples distortion in samples of natural (pseudo)orthorhombic examined during our current work. The table also ixiolite, indicative ofincipient ordering, was detected presentsthe acronyms used to designatelocalities in by the above authors, apparently converging with most of the figures and some of the tables below. In highly disordered pseudo-orthorhombic wodginite total, data on titanian columbite - tantalite and ixiolite fl.;ahn D82, Wise 1987, Ercit 1986, Ercit et aI. t992a). from four localities were exffacted from the literature, Thus, stannianixiolite is establishedas a disordered besides those from five localities examined in our counterpart of the ordered wodginite. laboratories and published over the last 17 years. Compositional variations led to recognition of Some of these data were in part complemented during additional varieties of ixiolite. namelv titanian ixiolite. the current work, which covered samples from 11 scandian ixiolite and "wolframo-ixiolite" (iernf & additional localities. Ercit 1985, 1989, Wise & Cernf 1986). Whereas the A broad compositional range is covered by the crystal-chemical and structural behavior of scandian selected samples, with the TiO, content of the ixiolite and "wolframo-ixiolite" remained essentially orthorhombic phase variable from -2 to -20 wt.Vo. uncharted, titanian ixiolite was generally considered a Both homogeneousminerals and exsolved intergrowths close analog of its Sn-rich countepart(e.5., eemg & with rutile are represented, as well as orthorhombic Ercit 1985, 1989). However, significant differences phases associated with rutile in a texturally non- were subsequentlyfound in the responseof Ti-rich replacive and unexsolved, but side-by-side coexisting samplesto heating; widespread presenceof associated manner. In some cases,the orthorhombic phase and rutile was noticed, and the need for a systematic study rutile are dispersedin the parent rock, but noi in mutual was recognized. contact; in others, rutile is absent. Here we present a review of data from the literature Care was taken to select samples of reasonably and our new observations,as a first step toward proper simple compositions,free of substantialquantities of characterization and definition of titanian derivatives other components that may influence the structural of the columbite - tantalite and ixiolite minerals, and behavior or phase relationships of the orthorhombic of their relation to the wodginite and rutile groups of mineral on heating. In this respect,the most significant minerals in the system TiO2 - (Fe,Mn)z*(M,Ta)s*rOu- element is Sn, which also is the most common one in Fern(Nb,Ta)Oo. the minerals examined. Only samples that show Ti/Sn (at.) greater than 2 were used here; unfortunately, PnrvlousWom Ti-rich specimens truly poor in Sn are quite rare. Scandian ixiolite, notorious for its high Ti content, was Information on the Ti-enriched members of the not employed. family of columbite minerals sensu lato (ordered to disordered columbite group, ixiolite, wodginite group) Expennmrrnr Mnrgops is so far rather meager, dispersed in papers describing individual occrurences,and is in most casesincomplete. Chemical composition was establishedby means Data on chemical composition are commonly not of electron-microprobeanalysis. Some of the data were accompaniedby structural characteristics(eemj et al. obtained using the MAC-5 instrument in energy- 1981a, Zagorskyi & Peretyazhko 1992, Johan & Johan dispersion mode, under conditions given in Cem! et al. 1994); this is due in part to a fine grain-size of the (1986). The other data were generatedin wavelength- samplesand dispersal in and intimate intercrowth with dispersion mode on the CAMECA SX-50 apparatus" othei phases((em! et al. 1986, dernf & NEmec 1995). using the sameanalytical conditions and standardization TITANIAN IXIOLITE AND TITANIAN COLUMBITE - TANTALITE 549 TABLE 1. LIST OF EXAMINED SAMPLES Locality Comp.# Associationr Source/Reference2 CentralTransbaikalia, Russia ZAG-I, ta -2 hom"nor Zagorskyi& Peretyazhko(1992) CfnovecoCzech
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